1. Field of the Invention
The present invention relates to an image reading method and an image reading apparatus for periodically reading an image on a document irradiated by a light emitting source in the main scanning direction by an image pickup sensor having a plurality of light receiving elements in a main scanning direction while relatively moving at least one of the light emitting source and the document against the image pickup element in a sub-scanning direction.
2. Description of Prior Art
There is an image reading apparatus for reading an image on a document irradiated by a light emitting source having a plurality of light emitting elements in the main scanning direction through an image pickup sensor, such as a line sensor, while relatively moving a light emitting source and the image pickup sensor against the document in the sub scanning direction.
In the image reading apparatus described above, there is an image reading apparatus having a reduction function for generating reduced size image data. With regard to the reduction function of the image forming apparatus, the technologies related to the reduction function are disclosed in FIG. 1 on page 1 of Japanese Unexamined Laid-Open Patent Publication No. 2000-224380.    (A) The conventional reduction function of the image reading apparatus has been executed by the method described below.(A1) Motor Control:
When simply executing the reduction function, a control section of the image reading apparatus controls the scanning velocity in the sub-scanning direction Sv′ to be inversely proportion to a magnification ratio α against the scanning velocity Sv at the real size operation. This will be expressed by a following formula (1).Sv′=Sv/α  (1)
For example, assume that the sub-scanning velocity Sv at the non-magnification equals to 465 mm/s, the scanning velocity in the sub-scanning direction Sv′ is going to be 930 mm/s when reducing a magnification ratio to 50%, because Sv′=465/0.5=930.
Graph (a) in FIG. 7 illustrates the scanning velocity in the sub-scanning direction when scanning in a reduction process with a motor control. Namely, since a read magnification (the magnification ratio) α is inversely proportion to the scanning velocity in the sub-scanning direction Sv′, when the reading magnification is set at 25%, which is the minimum magnification ratio, the scanning velocity Sv′ in the sub-scanning direction becomes four times of the scanning velocity in the sub-scanning direction set for the non-magnification. Accordingly, the workload on the motor and driving circuit is large and it influences on the life of the apparatus. Further to this, vibration is posed due to high speed driving; consequently, it becomes necessary to provide a vibration-absorbing member to solve these problems described above.
(A2) Motor Control+Data Thinning-Out Control:
Since the scanning velocity in the sub-scanning direction Sv′ comes high when only motor control is executed, actual operation and control of the image reading apparatus becomes difficult. Then when the magnification ratio falls in the range from 100% to 50%, the control section of the image reading apparatus executes the same control as the motor control of (A1). And when the magnification ratio falls in the range from 50% to 25%, the control section of the image reading apparatus executes data thinning-out control for thinning out the read data of every other line in the main scanning direction. Based on the data thinning-out control, when the magnification ratio falls in the range from 50% to 25%, the scanning velocity in the sub-scanning direction can be maintained in the same scanning velocity in the sub-scanning direction Sv′ when the magnification ratio falls in the range from 100% to 50% (refer to (b) in FIG. 7). Namely, when the magnification ratio is set at 25% being the minimum magnification ratio, the scanning velocity in the sub-scanning direction Sv′ becomes twice of the scanning velocity in the sub-scanning when the magnification ratio of real size. Consequently, the workload of the motor and driving circuit can be improved a little bit. However, due to the data thinning-out control, degradation of image quality occurs.
(A3) Motor Control+Data Thinning-Out Control+Extending Reading Cycle in the Main Scanning Direction:
In the case of the motor control+data thinning-out control described in (A2) above, it is necessary to increase the scanning velocity in the sub-scanning direction Sv′ twice of the previous scanning velocity when setting the magnification ratio from 50% to 25%. Accordingly, even though the vibration is reduced comparing with the case of (1), however actual operation and control become difficult and still there is a problem that vibration occurs. Consequently, a technique for extending the main scanning direction reading cycle (image pickup element charge accumulation time) of the image pickup sensor such as a line sensor is newly introduced. Namely, even though the scanning velocity in the sub-scanning direction is kept constant by extending the main scanning direction reading cycle (extension factor β), the number of lines of the reading data in the main scanning direction becomes 1/β. This means that the reduction effect, which is the same effect obtained when the sub-scanning velocity becomes β times can be obtained. Accordingly, when the magnification ratio is in the range from 100% to 75%, the motor control described in (A1) above is executed; when the magnification ratio is in the range from 75% to 50%, the motor control+reading cycle extension are executed; when the magnification ratio is in the range from 50% to 37.5%, the motor control+selective data elimination is executed; and when the magnification ratio is in the range from 37.5% to 25%, the motor control+reading cycle extension+data thinning-out are executed (refer to (c) in FIG. 7). Here, when the reading cycle extension β is assumed 1.5, the scanning velocity Sv′ in the sub-scanning direction becomes 1.5 times of the scanning velocity of a real size operation when the magnification ratio is 75%, 50%, 37.5% and 25% respectively. The minimum value of the scanning velocity in the sub-scanning direction is going to be as following when the extension factor β equal to 1.5. Since Sv′ min=(Sv/0.75)·(1/β)=0.89, the Sv′ min becomes 0.89 times of the scanning velocity of a real size operation.    (B) In the technique described above, according to the (A3), which utilize the extension of reading cycle, the scanning velocity Sv′ in the sub-scanning direction by the motor control can be regulated and the maximum scanning velocity in the sub-scanning direction becomes 1.5 time of the scanning velocity of a real size operation. However, the light receiving amount of the image pickup sensor becomes β times of the light receiving amount when normal operation. Accordingly, there is a possibility that the light receiving element included in the image pickup sensor is saturated. Accordingly, it is necessary to reduce the intensity of emitting lights of a light emitting element array for irradiating the document to be read to about 1/β when the image pickup sensor read the document.
In the case of controlling the intensity of emitting lights, since shading characteristic changes as the amount of emitting lights and/or exposing time changes, it is necessary to specially provide shading correction data for conducting shading correction.    (C) As described above, since the coverage to be covered by only motor control is so wide that various problems involving mechanical issue occur. On the other hand, even though various controls are combined to make the coverage narrow, still mechanical controls remain. Further, it is still necessary to use various electrical controls together with the optical system controls. Accordingly, there is still a problem that the controls become complicated.